The present invention relates to a scanning electron microscope, or in particular to a configuration and the control operation of a secondary electron detection system thereof.
The scanning electron microscope is a device in which the primary electron beam is radiated by scanning on a specimen and the secondary electrons generated from the specimen are detected by a secondary electron detector in synchronism with the scanning thereby to form a specimen image. Normally, secondary electrons having a very small energy on the order of several eV to several tens of eV are generated from the specimen irradiated with the primary electron beam, and applied to the secondary electron detector under the guidance of a transverse positive field of about 10 kV impressed from the secondary electron detector. In addition to this basic process, the scanning electron microscope recently developed further comprises a biased electrode impressed with a positive voltage of several hundred volts in the vicinity of the specimen to increase the yield by correcting the trajectory of the secondary electrons toward the detector. The biased electrode, tabular or net-shaped, is called the “secondary electron collector electrode” or the “auxiliary electrode” and is intended to increase the yield of the secondary electrons by changing the shape or the applied voltage to the optimum value. Various electrodes of this type intended to increase the yield of the secondary electrons by use of the positive field have been proposed, and it has been common practice to “correct the trajectory of the secondary electrons of negative potential by the positive field” (JP-A-6-103951, etc.). JP-A-9-147782, on the other hand, describes a method in which an annular electrode (U-shaped open to the side far from the secondary electron detector) impressed with a negative voltage of −100 to −500 V is arranged above a specimen to guide the secondary electrons toward the detector.
The secondary electrons generated from the specimen by the radiation of the primary electron beam thereon are emitted in various directions normally within the range of 0 to 90 degrees in vertical direction and 0 to 360 degrees in horizontal direction. In JP-A-9-147782, a U-shaped negative voltage application electrode open toward the secondary electron detector is arranged to guide the secondary electrons having various directivities toward the secondary electron detector. Analysis by simulation of the secondary electron trajectory has revealed, however, that the secondary electrons are not only absorbed by bombardment into the lower part or the substructure of a magnetic objective lens but also bombard the specimen stage and the various detectors (ground potential) arranged in the neighborhood of the magnetic objective lens, with the result that only a small part of the secondary electrons generated can reach the secondary electron detector. This trend is especially conspicuous in the case where the high-resolution observation is tried, i.e. the magnetic objective lens is close to the specimen, in which case an image of low quality results.